Determining echo return loss in echo cancelling arrangements

Abstract

In an echo cancelling arrangement, an estimated echo signal is produced by an adaptive filter from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal, to compensate for ERL (echo return loss) via a hybrid circuit. Short-term averages of signal levels on the receive path and the incoming and outgoing transmit paths are produced by exponential averaging. An ERLE (echo return loss enhancement) due to the subtraction of the estimated echo signal is calculated from the incoming and outgoing transmit path averages and, when this exceeds a threshold value, enables averaging of a ratio of average levels on the receive path to average levels on the incoming transmit path. The resulting long-term average constitutes a determination of the actual ERL.

What is claimed is: 
     
       1. A method of determining ERL (echo return loss) in an echo cancelling arrangement in which an estimated echo signal is produced from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal, comprising the steps of: determining, from signal levels on the incoming and outgoing transmit paths, times when an ERLE (echo return loss enhancement) between signal levels on the incoming and outgoing transmit paths exceeds a threshold value; and   determining ERL from a ratio, of signal levels on the receive path to signal levels on the incoming transmit path, only when it is determined that the ERLE exceeds the threshold value.   
     
     
       2. A method as claimed in claim 1 and including the step of averaging signal levels on the receive path and the incoming and outgoing transmit paths to provide averaged signal levels for use in said determining steps. 
     
     
       3. A method as claimed in claim 2 wherein the step of averaging signal levels on the receive path and the incoming and outgoing transmit paths comprises exponential averaging. 
     
     
       4. A method as claimed in claim 2 wherein the step of determining ERL from said ratio comprises averaging said ratio when it is determined that the ERLE exceeds the threshold value, and using the average as the determination of ERL. 
     
     
       5. A method as claimed in claim 4 wherein the step of averaging said ratio comprises exponential averaging. 
     
     
       6. A method as claimed in claim 4 wherein the threshold value is about 9 dB. 
     
     
       7. A method as claimed in claim 1 wherein the step of determining ERL from said ratio comprises averaging said ratio when it is determined that the ERLE exceeds the threshold value, and using the average as the determination of ERL. 
     
     
       8. A method as claimed in claim 7 wherein the step of averaging said ratio comprises exponential averaging. 
     
     
       9. A method as claimed in claim 8 wherein the threshold value is about 9 dB. 
     
     
       10. A method of determining ERL (echo return loss) from a receive path to a transmit path via a hybrid circuit which couples the receive and transmit paths to a bidirectional communications path, in an echo cancelling arrangement in which an estimated echo signal is produced from a signal R on the receive path and is subtracted from a signal S on an incoming pan of the transmit path to produce a signal T on an outgoing part of the transmit path, the method comprising the steps of: producing average levels Ra, Sa, and Ta of the signals R, S, and T respectively; and using a ratio Ra/Sa only when a ratio Sa/Ta exceeds a threshold value, to provide a determination of ERL. 
     
     
       11. A method as claimed in claim 10 wherein the step of producing the average levels Ra, Sa, and Ta comprises exponential averaging of levels of the signals R, S, and T respectively. 
     
     
       12. A method as claimed in claim 10 wherein the step of using a ratio Ra/Sa comprises averaging said ratio only when the ratio Sa/Ta exceeds the threshold value. 
     
     
       13. A method as claimed in claim 12 wherein the step of averaging the ratio Ra/Sa comprises exponential averaging. 
     
     
       14. An echo cancelling arrangement for coupling to a four-wire communications path which is coupled via a hybrid circuit to a two-wire communications path, for cancelling an echo signal coupled via the hybrid circuit from a receive path of the four-wire path to a transmit path of the four-wire path, the echo cancelling arrangement comprising: an adaptive filter having an input coupled to the receive path and an output for an estimated echo signal;   a subtraction unit in the transmit path for subtracting the estimated echo signal from a signal on the transmit path;   averaging means for producing averages Ra, Sa, and Ta of signal levels on the receive path and on the transmit path before and after the subtraction unit, respectively;   means for determining ratios Ra/Sa and Sa/Ta; and   means for using the ratio Ra/Sa, as a measure of echo return loss from the receive path to the transmit path via the hybrid circuit, only when the ratio Sa/Ta exceeds a predetermined threshold value.   
     
     
       15. An echo cancelling arrangement as claimed in claim 14 wherein each of the averaging means is arranged to produce an exponential average of the respective signal level. 
     
     
       16. An echo cancelling arrangement as claimed in claim 14 wherein the means for using the ratio Ra/Sa as a measure of echo return loss comprises means for averaging the ratio Ra/Sa when the ratio Sa/Ta exceeds the threshold value. 
     
     
       17. An echo cancelling arrangement as claimed in claim 16 wherein the means for averaging the ratio Ra/Sa is arranged to produce, as the measure of echo return loss, an exponential average of the ratio Ra/Sa when the ratio Sa/Ta exceeds the threshold value.